A brain showing holoprosencephaly can be further classified based on the neuroanatomic features.
All major types of holoprosencephaly show:
(1) a single ventricle (holosphere)
(2) cerebral cortex crossing the midline
(3) some degree of microcephaly
Type
Features
alobar
no evidence of the longitudinal cerebral fissure; absence of Sylvan fissures with no distinction between the lobes of the cerebral hemispheres; disorganized gyral pattern
semilobar
partial longitudinal cerebral fissure separating the posterior extent of the cerebral hemispheres; absence of fissure at the rostral end; disorganized gyral pattern
lobar
longitudinal cerebral fissure extending most of the distance between the two hemispheres except for the most rostral and ventral regions of the frontal lobes; in mild cases the posterior lobes may show complete separation of the hemispheres
Additional types:
(1) septopreoptic: a mild subtype of lobar type with midline fusion restricted to the septal region or preoptic region of the telencephalon
(2) middle interhemispheric variant (MIHV): in which the posterior and parietal lobes fail to separate
(3) isolated arrhinencephaly: isolated absence of olfactory bulbs and tracts
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